1. Field of the Invention
The present invention relates to an electrophotographic apparatus using a two-component developer in which toner and a carrier which is magnetic particles are mixed, and a developing device and a developing roll used therein.
2. Description of the Related Art
In an electrophotographic apparatus, a photoconductor is charged and exposure according to image data is performed and an electric charge distribution corresponding to an image pattern is formed on the photoconductor and an image appears as a visible toner image first by developing toner according to the electric charge distribution using a developing device. Thereafter, the toner image is transferred to paper and is fixed on the paper by thermal fixing and is completed as the image. In this development, there is a development method using two-component developer in which toner which is coloring particles of a resin powder with a particle size of the order of 10 xcexcm and a carrier which is magnetic particles such as ferrite, magnetite or an iron powder with a particle size of 50 to 150 xcexcm are mixed.
Also, in the developing device, a magnet is provided in the inside and a developer is carried in a development part which is a gap between the photoconductor and a developing roll by the developing roll whose external cylinder rotates. In this case, a regulating member for uniforming the amount of carriage of the developer in a length direction of the developing roll is provided within the developing device. The regulating member is referred to as a doctor blade and a distance between the doctor blade and the developing roll is referred to as a doctor gap. Also, a distance between the photoconductor and the developing roll is referred to as a development gap.
By the way, images formed by an electrophotographic method include a character, a line drawing, a dot or solid matter. With respect to all of these images, it is necessary to develop a proper amount of toner by the developing device so as to become an image with a recognizable density.
However, the actual development has characteristics in that the amount of development of toner becomes small in the inside of a solid image and the amount of development becomes large in the circumference portion of the solid image or portions close to the boundary with white fields of the line drawing or the dot. This is because a development electric field becomes strong in the boundary portions with the white fields by a peripheral effect. Here, the development electric field means an electric field of a direction perpendicular to a surface of the photoconductor determined by an electric charge distribution of the photoconductor surface or a development bias voltage applied to the developing roll, a film thickness and a dielectric constant of the photoconductor, a development gap and a dielectric constant of the developer, and toner particles charged by this electric field are attracted or replused and thus the development is performed.
This peripheral effect becomes a cause and in the case of setting development conditions (for example, a development bias voltage, a toner density, a peripheral velocity ratio) in which the amount of development of toner as the solid image becomes a proper density is obtained, the amount of adhesion becomes excessive in the line drawing or the dot and a scattering at the time of transfer becomes large, or a distortion of an image due to melting of toner at the time of fixing occurs.
For this reason, the circumference of the image or the line drawing becomes obscure, or a photographic image formed of dots becomes unclear due to a distortion or becomes a solid image even though a dark portion of the image is not perfectly solid in the extreme case.
Since the problems described above are due to the peripheral effect of the development electric field, it is possible to avoid the problems in principle by setting development conditions so that the peripheral effect is eliminated.
One of the avoidance means is means for narrowing a development gap described in JP-A-56-161565.
This conventional example relates to a technique for avoiding a problem that the circumference of an image is edged with another color when a two-color image is formed, and a cause of the problem is similar to the peripheral effect of the development electric field becoming the problem in the present application.
The reason why this peripheral effect can be reduced by narrowing the development gap is because a developing roll which is an actual electrode approaches a photoconductor and thereby an electric field between the photoconductor and the developing roll is determined by a development gap which is a gap between them and a development potential difference between a development bias voltage and a photoconductor potential. This phenomenon is shown in FIGS. 3A and 3B. This was found from a result of calculating an electric field strength distribution of a photoconductor surface in the case of keeping an electric charge distribution on the photoconductor constant and changing the development gap that an electric field strength of the solid inside increases by narrowing the development gap and approaches an electric field strength of the solid circumference or the line drawing as shown in FIG. 3B. Also, it could be recognized from the fact that when the development gap is actually narrowed, the amount of toner development of the solid inside increases and an image density becomes high.
By the way, in the conventional example, the amount of carriage of a developer is secured by providing projections and depressions on a surface of the developing roll and the peripheral effect is controlled by narrowing the development gap in the projections of the surface.
On the other hand, as a problem to which attention must be paid in the case of narrowing the development gap, when the development gap is narrowed, a volume density of the developer between the photoconductor and the developing roll becomes high and a pressure to the developer or a force rubbing the photoconductor increases. When this force becomes excessive, the developer starts to harden or the photoconductor is damaged. Actually, toner is resin and hardens when pressed by an excessive force, so that the toner performs a function like an adhesive as the developer and hardening of the developer including a carrier is caused and this is referred to as a developer lock. Further, this hardening becomes a cause and the photoconductor may be damaged.
As a result of that, in the case of narrowing the development gap, it is constructed so that a doctor gap is also narrowed at once and a volume density of the developer does not become too high between the photoconductor and the developing roll. Actually, the proportion of the volume including the toner and the carrier must be set to less than or equal to 50% the volume of space between the photoconductor and the developing roll.
However, in the case of using the developing roll of the conventional example or conventional developing rolls other than the developing roll, it is necessary to set the doctor gap narrower than the development gap, and in order to reduce the excessive development of the solid image circumference, the line drawing, the dot,due to the peripheral effect becoming the problem in the present application, a need to narrow the doctor gap further arose when the development gap attempts to be shortened further.
But, when the doctor gap; is narrowed, it proved that the amount of the developer passed from a doctor blade becomes non-uniform in a length direction of the developing roll. This results from variations in a flexure of the doctor blade or a surface shape of the developing roll and it is considered that the non-uniformity occurs because their variations become large relatively by narrowing the doctor gap. Therefore, in a method necessary to narrow the doctor gap more than the development gap in this conventional example or other conventional art, it is difficult to solve the problem to be solved in the present application still.
The invention has been made to solve such a problem that the amount of carriage of a developer does not become uniform in a length direction of a developing roll when the amount of carriage of the developer on the developing roll is decreased as described above, and a narrow development gap is implemented by solving this problem, and therefore an object of the invention is to provide an electrophotographic apparatus capable of reducing an excessive amount of toner adhesion to an image area of a solid image circumference, a line drawing or a dot due to a peripheral effect and obtaining a sharp image without distortions.
In order to solve the problem, grooves of a length direction of a developing roll are formed on a surface of the developing roll and a magnetic pole of the inside of the developing roll is provided in a position of a doctor blade for regulating the amount of carriage of a developer roll and further a magnetic force on the surface of the developing roll of the position of the doctor blade is set to 0.06 T or less.
Further, a ratio of a groove depth to a groove width is set to one or more, and a ratio of a groove pitch to a groove width is set to two or more, and a groove depth is set to three times or more a carrier particle size.